Droplet generation in a microfluidic device having an optoelectrowetting configuration

Abstract

Systems and methods are described herein for improved droplet generation within microfluidic apparatuses. Electrowetting forces of varying configurations may be used to separate droplets from a fluidic reservoir in a reproducible and rapid manner. In many embodiments, separation of droplets from the fluidic reservoir is performed without the use of highly specialized surfactants.

What is claimed: 
     
       1. A method of generating a droplet in a microfluidic device comprising a substrate and an optoelectrowetting (OEW) configuration, the method comprising:
 applying a first electrowetting (EW) force to a droplet reservoir disposed within the microfluidic device, wherein the droplet reservoir comprises an aqueous fluid; 
 projecting an optical droplet actuator, wherein the optical droplet actuator comprises a projected pattern of light sufficient to activate a second electrowetting force, onto a first position on a surface of the substrate of the microfluidic device, wherein the first position overlaps at least partially with a position on the surface of the substrate that is in contact with the aqueous fluid of the droplet reservoir; and 
 moving the projection of the optical droplet actuator to a second position on the surface of the substrate of the microfluidic device, wherein the second position is a sufficient distance away from the first position so as to cause a first droplet of the aqueous fluid to separate away from the droplet reservoir, 
 wherein, prior to moving the projection of the optical droplet actuator to the second position on the substrate, the droplet reservoir contains a volume of aqueous fluid equal to or greater than twice the volume of the first droplet. 
 
     
     
       2. The method of  claim 1 , wherein the first droplet has a volume V 1  that is proportional to a cross-sectional area A ODA  defined by the optical droplet actuator. 
     
     
       3. The method of  claim 2 , wherein the volume V 1  is provided by the equation:
     V   1 =( A   ODA   *H   E )*(1+ P   1 ), 
 wherein H is a cross-sectional height of a chamber in the microfluidic device in which the droplet reservoir is disposed, and 
 wherein P1 ranges from 0.00 to 0.25. 
 
     
     
       4. The method of  claim 3 , wherein the cross-sectional area AODA is about 25,000 to about 250,000 microns. 
     
     
       5. The method of  claim 1 , wherein the first droplet has a volume V 1  of at least 1 nL to about 10 nL. 
     
     
       6. The method of  claim 1 , wherein the optical droplet actuator comprises a first portion having a leading edge and a second portion having a trailing edge. 
     
     
       7. The method of  claim 6 , wherein the leading edge of the first portion is convex or the leading edge of the first portion is substantially straight. 
     
     
       8. The method of  claim 6 , wherein the second portion is tapered or the trailing edge of the second portion tapers to a single vertex. 
     
     
       9. The method of  claim 8 , wherein a length of the second portion ranges from about 100 microns to about 1000 microns. 
     
     
       10. The method of  claim 6 , wherein the first portion and the second portion of the optical droplet actuator are portions of a single contiguous optical droplet actuator. 
     
     
       11. The method of  claim 6 , wherein the first portion and the second portion of the optical droplet actuator are separate portions of a composite optical droplet actuator. 
     
     
       12. The method of  claim 1 , wherein the droplet reservoir comprises a volume of the aqueous fluid of at least 2.5 nL. 
     
     
       13. The method of  claim 1 , wherein at least a portion of the EW force applied to the droplet reservoir remains in a stationary position as the optical droplet actuator is moved toward the second position and away from the droplet reservoir. 
     
     
       14. The method of  claim 1 , further comprising:
 modifying the optical droplet actuator responsive to generating the first droplet, wherein modifying the first optical droplet actuator comprises expanding the area circumscribed by the projection of the optical droplet actuator while the projection of the optical droplet actuator is moved toward the second position and away from the droplet reservoir or after the projection of the optical droplet actuator has reached the second position. 
 
     
     
       15. The method of  claim 1 , wherein applying the EW force to the droplet reservoir comprises applying an electrical potential across opposing electrodes of the microfluidic device, wherein the electrical potential has a voltage of about 20 Vppk to about 45 Vppk. 
     
     
       16. The method of  claim 15 , wherein the applied electrical potential has a current having a frequency of about 10 kHz to about 100 kHz. 
     
     
       17. The method of  claim 1 , wherein the aqueous fluid of the droplet reservoir comprises a surfactant, wherein the surfactant is selected from the group consisting of N-(1,3-bis(Glucopyranoside)propan-2-yl)-3-Butyl-3-Phenylheptanamide and 2, 4, 7, 9-Tetramethyl-5-decyne-4,7-diol ethoxylate. 
     
     
       18. A system configured to generate droplets of aqueous fluid within a microfluidic device, the system comprising:
 a nest; 
 a structured light modulator (SLM); 
 an optical train; and 
 a control module comprising a digital processor and a digital memory, 
 wherein the nest is configured to support the microfluidic device; 
 wherein the optical train is configured to receive light from the SLM and project an optical droplet actuator onto a surface of a substrate of the microfluidic device when the microfluidic device is supported by the nest; 
 wherein the digital memory of the control module comprises non-transitory machine readable instructions for carrying out the steps of the method of  claim 1 ; and 
 wherein the processor of the controller is configured to read the machine-readable instructions from the memory and, in accordance with the instructions, direct the SLM to project an optical droplet actuator onto a first position on a surface of a substrate of the microfluidic device and move the projection of the optical droplet actuator from the first position to a second position on the surface of the substrate of the microfluidic device. 
 
     
     
       19. The system of  claim 18 , wherein the nest is further configured to electrically couple with and apply an electrical potential across the microfluidic device. 
     
     
       20. The system of  claim 19 , wherein the digital processor is configured to direct the nest to apply the potential across the microfluidic device, and wherein the digital processor is configured to receive user input to determine the voltage potential to apply across the microfluidic device. 
     
     
       21. The system of  claim 18 , further comprising an imaging device configured to provide a digital image of at least a portion of the microfluidic device comprising a droplet reservoir. 
     
     
       22. The system of  claim 21 , wherein the digital processor is configured to receive the digital image of the portion of the microfluidic device, analyze the image to identify a position on the surface of the substrate of the microfluidic device that is in contact with an aqueous fluid of the droplet reservoir, and select the first position such that the first position at least partially overlaps with a portion of the identified position that is in contact with the aqueous fluid. 
     
     
       23. The system of  claim 21 , wherein the digital processor is configured to receive user input to select the first position. 
     
     
       24. The system of  claim 22 , wherein the digital processor is further configured to estimate an area of the surface of the substrate contacted by the droplet reservoir and, based upon the estimate, actuate an electrowetting (EW) force within the microfluidic device to pin the droplet reservoir. 
     
     
       25. The system of  claim 24 , wherein the digital processor is configured to dynamically adjust the EW force used to pin the droplet reservoir as a first droplet is separated from the droplet reservoir or after the first droplet is separated from the droplet reservoir and before a second droplet is separated from the droplet reservoir. 
     
     
       26. The system of  claim 18 , wherein the digital processor is configured to dynamically adjust a cross-sectional size of the optical droplet actuator as a first droplet is separated from the droplet reservoir.